54 PEOPAGATION OF PLANTS. 



the rapid transmission of fluids through the succulent 

 stems of herbaceous plants and the young plantlets, and 

 twigs of shrubs and trees. The solidity of the stems of 

 the trees diminishes from the center to circumference as 

 they increase in size, or just the opposite of what takes 

 place in endogenous stems, the inner portions, in time, 

 ceasing to take any actiye part in the movement of the 

 outer layers, and the heart-wood may decay, as seen in 

 thousands of instances in almost every old forest, or be 

 forcibly removed without severely checking the growth 

 of the younger parts of the tree. But so long as the 

 center of the stem remains entire, there is a slight com- 

 munication between the outer and adjacent parts through 

 what are called the medullary rays, which are composed 

 of cells spread out into a thin membranous structure. 

 These rays are quite abundant and conspicuous in the 

 wood of the Beech, Maple, Oak and many other kinds of 

 trees. Through these rays the inner parts of the stem 

 are supposed to be supplied with sufBcient moisture to 

 prevent complete exsiccation of the wood. 



Following in the same direction, from the pith outward 

 through the mature wood, we next reach a later forma- 

 tion composed of a few or many concentric layers which, 

 as a whole, are called alburnous wood, or alburnum, from 

 the Latin alhus, white, because in most trees and shrubs 

 this recently formed wood is of a whitish color, or at 

 least lighter in color than the duramen or heart wood, 

 and the cells of which this alburnum is composed are 

 capable of transmitting living, organizable matter. The 

 cells of the inner layers of alburnum are, however, less 

 active than those of the outer, and the propagator of 

 plants by division, finds the latest formed wood responds 

 most readily to his wishes m the various operations to 

 which it is subjected. Encircling the alburnum we 

 find a layer of soft organizable matter which has received 

 the name of cambium, which is quite abundant in some 



